The invention disclosed herein pertains to a filter assembly for filtering coolant air that passes through an avionic line replaceable unit (LRU) box. The invention is an improvement over copending application Ser. No. 08/033,158, filed Mar. 16, 1993. The copending application is assigned to the assignee of the present application.
In aircraft, electronic and electrical circuitry is customarily contained in LRUs which are essentially boxes that have air inlet and outlet holes on their top and bottom surfaces. The LRUs are installed in trays whose dimensions and configurations are standardized in accordance with the requirements of the avionic industry. Multiple circuit electrical connectors are typically mounted in the rear part of the tray and mating electrical connectors are mounted on the rear of the LRUs so that when the LRU is slid into the tray, the mating connectors join and a plurality of electrical circuits are completed. The bottom or base of the tray is generally planar and has a large opening, typically, for communicating the air passing into or out of the LRU with a shelf constituting a plenum whose interior is at negative pressure relative to atmospheric pressure. An opening in the base of the tray has a gasket that effects a seal with the bottom of the LRU box and with the underlying plenum shelf. The opening in the plenum shelf is typically subdivided into a plurality of openings which can be selectively plugged or sealed to accomplish what is called metering of the air flow through the LRU box. In dedicated designs use a fixed number of metering holes their total cross-section to provide the air flow volume required in a particular LRU.
In existing aircraft, the customary practice is to maintain the plenum or shelf interior at negative pressure such that coolant air enters or is drawn in through the top holes of the LRU box and exits or is drawn out through the holes in the bottom of the box. This air is conducted away and possibly discharged to the atmosphere through a conduit in which there is a suction or pressurizing fan that maintains a desired negative pressure condition in the plenum.
The LRUs are mounted in avionic trays in various compartments of the aircraft such as in a compartment beneath the pilot's cabin or in a compartment located above or below the passenger cabin. The cooling air for the LRUs is drawn into compartments of the aircraft by various means including ventilation systems, ventilation ports, doors, hatches and the like and is discharged either into the compartment or elsewhere by way of the plenum or shelf air conduit. Avionic trays may be installed in compartments which are in air circulation communication with the passenger compartment, the baggage compartment and other subcabin compartments wherein the air may contain various contaminants such as smoke, grease, aerosols, insects, hair and other organic matter in aircraft which carry live animals, for example. These contaminants are entrained in the coolant air that is either forced through or drawn through the LRUs. The result is that the electronic components in the LRUs, such as circuit boards or circuit elements, acquire a coating of contaminants which, in some cases are conductive and in any case constitute a barrier for heat dissipation. The operating life of electronic circuits is correlated to the temperature at which they operate. When dirty or contaminated air is circulated through the LRU boxes, failures of the circuitry occur prematurely, and such failures, needless to say, may jeopardize the safety of the aircraft and its passengers, as well as being very costly for the aircraft operator.
Replacing an LRU involves more than simply unplugging one unit and inserting a substitute unit into the avionic tray. If the connection between the multiple pin, plug and socket connectors on the LRU and on the avionic tray are separated, government and avionic maintenance industry rules require checking the integrity and function of every one of the plurality of circuits that are made by the multiple line connectors before the replacement process can be accepted, and the aircraft can be restored to operation. It will be evident, therefore, that it would be highly desirable to have a coolant air filter assembly associated with each LRU that not only provides for filtering the input air of the LRU and assures uniform airflow through the LRU, but also allows replacement of the dirty or contaminated filter without requiring that the LRU be disconnected, withdrawn from the avionic tray or otherwise disturbed.
The air filtering system for an LRU described and claimed in the above-cited copending application has achieved the objectives set forth in the preceding paragraph and more. The copending application discloses how to construct a filter assembly that is adapted for exchanging filter elements without requiring withdrawal of the LRU from the avionic tray or otherwise disturbing the connection of the LRU box to the connector on the tray. Moreover, in the filter assembly disclosed in the copending application, the parts are arranged such that a plenum is defined between the bottom of the filter element and the planar top surface of the LRU box to enhance distribution of the air over the top surface and, hence, uniformly over the cross-sectional area of the interior of the LRU box. One possible disadvantage of the previous filter assembly design is that it requires fastening an auxiliary frame structure onto the avionic tray to support the filter assembly from the tray. Adding parts to a conventional avionic tray is not inconvenient in the factory where the frame can be mounted to the avionic tray permanently, but on existing aircraft where LRUs are mounted in conventional fashion on the avionic trays, it is not practical to remove the trays and modify them by adding an auxiliary filter assembly supporting frame.